The overall objective of the proposed research is to continue to define and characterize the molecular mechanisms underlying genetically-determined resistance and susceptibility in snaims Biomphalaria glabrata to infection by larval Schistosoma mansoni, causative agent of human schisstosomiasis mansoni. Genetically-defined susceptible (PR albino) and refractory (10-R2) stocks of B. glabrata will be emloyed in comparative immunological and biochemical studies designed to provide an in-depth analysis of the three major interacting components involved in the immune association between S. mansoni and its snail host: the circulating hemocyte, soluble hemolymph components, and the schistosome tegumental surface. Specifically, monoclonal antibody and lectin probe techniques, in conjunction with in vitro cultivation methods will be employed to identify and characterize molecularly distinct hemocyte populations and assess the role of isolated components in mediating parasite recognition and host cell reactivity, to analyse the antigenic composition of the larval schistosome surface with emphasis on the possible involvement of shared or acquired host antigens in determining immune compatibility, to determine the mechanism(s) of snail hemocyte cytotoxic killing of schistosome sporocysts in vitro, and to assess the relative importance of cellular and humoral components in mediating immune reactions to schistosome larvae in vivo using adoptive transfer methods.